Ideas for how to make a cube explode into smaller cubes in game engine
I did this in a way that is not computationally expensive for physics with the "Bake to keyframe" option.

I make my basic set up in Blender Render. The eight cubes that will compose the fractured cube are all set to rigid body "active" objects. A plane is added below them, and set to rigid body "passive".

Next, I add the object that will force the cubes apart, it can be anything, a monkey, a cube, ect... ect... Place it at the junction between the cubes and make it passive like the ground plane.

Now, play the animation through to make sure you like it. Scaling the "exploder" object has great effect on how the cubes break apart. 

Select all the cubes and SpacebarSearch + "bake to keyframes". Click it, then click "Okay" on the popup window.

Now all you need to do is make each cube play the animation(s) always in game, parent them all to another no-collision object, move to another layer, and have the original cube add the parent when it dies.

If you create more than one such animation and use a Random sensor you can have great looking, variable, computationally inexpensive exploding cubes for your game.